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Black hole behind a cosmic ring of dust

Researchers observe the centre of the active galaxy NGC 1068

At the heart of the galaxy NGC 1068 lurks a supermassive black hole, hidden behind a cosmic dust cloud. Using the Very Large Telescope of the European Southern Observatory (ESO), an international team of astronomers has unveiled this supermassive gravity trap. The researchers, including scientists from the Max Planck Institutes for Astronomy and Radio Astronomy, gained new insights into the mechanisms of active galactic nuclei, some of the brightest and most enigmatic objects in the universe. They also confirmed a 30-year-old theory.

Active Galactic Nuclei (AGN) are extremely energetic sources powered by supermassive black holes located at the centre of certain galaxies. The central black holes are fed by large amounts of cosmic dust and gas. Under the spell of gravity, the material spirals towards the mass monster. Enormous amounts of energy are released in the process, often literally dwarfing the light of all the stars in the galaxy.

Since the discovery of these bright objects in the 1950s, astronomers have been puzzling over the processes involved. Now, a group led by Violeta Gámez Rosas from Leiden University in the Netherlands has come a decisive step closer to understanding these AGN.

The team used the high-power Matisse instrument on ESO's Very Large Telescope to observe the centre of the galaxy NGC 1068 in the constellation Cetus at infrared wavelengths in the range of 3 to 12 micrometres. The researchers detected a thick ring of cosmic dust and gas hiding a supermassive black hole.

This discovery is important evidence for a 30-year-old theory that is considered the standard model for AGN. This is because there are different types of these active galactic nuclei. While some shine brightly in visible light, others, such as the galaxy NGC 1068, appear to be rather suppressed in optical light. According to the Standard Model, all AGN have the same basic structure despite these differences: a supermassive black hole surrounded by a thick ring of dust.

According to the theory, the difference in the appearance of the AGN depends on the orientation with which we view the black hole and its thick dust ring from Earth – in other words, how much the ring obscures or even completely covers the black hole from our view.

In the case of the galaxy NGC 1068 – also known as Messier 77 or M 77 – 47 million light years away, researchers had already found evidence to support the Standard Model earlier, including the discovery of warm dust at the centre. However, doubts remained as to whether this dust could fully hide a black hole, explaining why AGNs of this type shine less brightly in visible light than others.

"The real nature of the dust clouds and their role in both feeding the black hole and determining how it looks when viewed from Earth have been central questions in AGN studies over the last three decades",says team leader Violeta Gámez Rosas. The results now obtained could lead to a better understanding of how AGN works and also help us to better understand the history of our Milky Way, which harbours a supermassive black hole that may have been very active in the past.

With the Matisse instrument, the astronomers have now gained a detailed picture of the shape and structure of the cosmic dust in the centre of NGC 1068: it appears as a thick inner ring and an extended outer disk. The observations also made it possible to determine the position of the black hole fairly accurately. All in all, the results support the standard model described.

"We established the position of the black hole from the combination of the infrared intensity distribution, the temperature distribution of the dust, determined for the first time, and with the help of images in the radio wave range," says Gerd Weigelt, director at the Max Planck Institute for Radio Astronomy in Bonn. His colleague Thomas Henning, director at the Max Planck Institute for Astronomy in Heidelberg, adds: "With Matisse, we were able for the first time to obtain images in the thermal infrared with a spatial resolution 10 times higher than before."

And: "The improved performance is crucial for confirming the assumption that the active core of the galaxy NGC 1068 is indeed surrounded by a dust torus," says Henning. His institute was instrumental in building the spectro-interferometer, and the Max Planck Institute for Radio Astronomy also contributed.

Gerd Weigelt casts a glance into the future: "Future observations with the Matisse instrument with low and additionally also high spectral resolution will allow us to study the structure of active galactic centres at various important wavelengths."